1. Field of the Invention
The present invention relates to a method for manufacturing a hydrogen absorbing alloy electrode, and more particularly, to a method for manufacturing a hydrogen absorbing alloy electrode for use in a cylindrical secondary cell.
1. Description of Related Art
A nickel-metal hydride secondary cell, which has a high energy storage capacity, typically comprises a negative electrode made of hydrogen absorbing alloy for absorbing and releasing hydrogen (more generally, a negative electrode active material), and a positive electrode comprised of an electrically conductive base member which carries nickel monoxide (more generally, a positive electrode active material). These electrodes are disposed in alkali electrolyte. A cylindrical type of nickel-metal hydride secondary cell comprises a cylindrical enclosure, and a power generation element accommodated therein, the enclosure having an auxiliary function of a negative electrode terminal. The power generation element, comprised of a positive electrode which is coiled in a spiral form together with a negative electrode piled on the positive electrode via a separator, is disposed on an insulating plate which is in turn disposed on the bottom of the cylindrical enclosure. The separator carries alkali electrolyte. Further, a cap, having an auxiliary function of a positive electrode terminal, is disposed on the power generation element through an insulating plate, etc. Preferably, the positive and negative electrodes are so constructed as to provide a large energy storage density, and prevent separation of the positive electrode active material and the hydrogen absorbing alloy therefrom when these electrodes are coiled.
From this point of view, generally used is a negative electrode sheet which is fabricated by applying a slurry of hydrogen absorbing alloy onto an electrically conductive sheet such as a punched metal sheet, and by drying the sheet to which the slurry has been applied. This negative electrode sheet is cut into negative electrodes. The punched metal sheet is fabricated by forming a plurality of apertures in a nickel sheet or by effecting nickel plating or nickel vacuum evaporation onto a sheet formed with a plurality of apertures, for instance. The alloy slurry is prepared with use of hydrogen absorbing alloy powder, thickener such as methylcellulose, and water obtained by ion-exchange.
In order to satisfy requirements such that the punched metal sheet must have suitable flexibility, electric conductivity, etc., the aperture rate, i.e., the ratio of the total area of the apertures to the total area of the sheet, must fall within an appropriate range. Once the aperture rate is determined, a suitable number of apertures and a suitable aperture diameter, i..e., a suitable aperture array arrangement in the punched metal sheet is also determined. Generally adopted is a staggered arrangement where the apertures are arranged in a zigzag fashion, so that lines connecting the center points of adjacent three apertures constitute an equilateral triangle, for instance.
Further, the apertures are typically arranged such that, when the punched metal sheet is projected onto an imaginary plane which extends perpendicularly to the sheet and which contains a sheet axis extending in the longitudinal or width direction of the sheet, projected apertures overlap one another, whereas projected aperture-non-formed portions of the sheet are present between adjacent projected apertures when the sheet is projected onto another imaginary plane extending perpendicularly to the sheet and containing the other axis (hereinafter referred to as the second axis) of the sheet.
However, when a negative electrode sheet fabricated from a punched metal sheet having the aforementioned aperture arrangement is coiled, especially, in the direction of the second axis, the thus coiled negative electrode sheet is sometimes cracked along the second axis direction at those locations in the second axis direction at which the aperture-non-formed portions are present. It can be considered that this results from poor bending strength of the negative electrode at its aperture-non-formed portions in which a smaller amount of the hydrogen absorbing alloy powder is filled than that of the powder filled in its aperture-formed portions, so that the stress produced In the negative electrode during the coiling of the electrode concentrates at the aperture-non-formed portions concerned. If a crack is formed in the negative electrode, a layer of the alloy powder adhered to the surface of the negative electrode breaks through the separator to be brought into contact with the positive electrode, resulting in a short-circuited failure of a secondary cell accommodating therein these electrodes.